The major goal of this research program is to identify the gene products of Friend and Rauscher spleen focus forming viruses (SFFV), and to obtain evidence concerning their leukemogenic roles. One approach has been to clone these SFFVs into fibroblasts and to compare their gene products. Our work already accomplished suggests that these SFFVs are closely related to each other and to the dual tropic MuLVs which are env gene recombinants of ecotropic and xenotropic viruses. Although F-SFFV and R-SFFV have nearly identical recombinant env genes, they differ dramatically in their gag and pol genes. The R-SFFV genome encodes intact gag polyproteins and enzymatically active reverse transcriptase. We have obtained evidence that the gp55s encoded by SFFVs are heterogeneously metabolized due to a structural instability which results in efficient processing into the plasma membranes of infected cells. Recently we have succeeded in isolating two F-SFFV mutants with gp55 structural gene abnormalities, and we are currently analyzing the leukemogenic properties of these viral mutants.